1. Field of the Invention
The present invention relates to an image display apparatus that displays images by utilizing electron beams.
2. Related Background Art
As flat-plate type display apparatuses, a liquid-crystal display apparatus, EL display apparatus, an and plasma display panels have been hitherto put into practical use. They, however, are on a level unsatisfactory for image display in view or their visual field angle, color display, brightness and so forth. In particular, compared with cathode ray tubes (CRT), there is a great difference in display performance, and they are still not substitutive for the cathode ray tubes.
Meanwhile, as information processing by computers has become higher in grade and television broadcasting has become higher in image quality, there is a rapid increase in a demand for high-precision and large-screen flat-plate type display apparatuses.
Accordingly, several proposals have been made on electron-beam acceleration flat-plate type display apparatuses for their use in image display. For example, as disclosed in U.S. Pat. Nos. 3,408,532 and No. 3,935,499 and Japanese patent Application Laid-open No. 56-28445, such apparatuses comprise a flat-plate type thermal electron emitting source, where an electron beam is emitted from this thermal electron emitting source and is controlled and accelerated by means of a group of control electrodes provided with a number of apertures corresponding with fluorescent material picture elements, with which electron beam a flat fluorescent screen is irradiated so that light is emitted from the desired fluorescent material picture elements.
In place of the above thermal electron emitting source, it is also attempted to produce flat-plate type display apparatus making use of a cold cathode as an electron emitting source.
First, conventionally known cold-cathode electron emitting sources include field emission types (hereinafter abbreviated "FE" types), metal/insulating layer/metal types (hereinafter abbreviated "MIM" types) and surface conduction type electron emitting devices (hereinafter abbreviated "SCE"). Examples of the FE types are disclosed in W. P. Dyke & W. W. Dolan, "Field Emission", Advance in Electron physics, 8, 89 (1956) and C. A. Spindt, "physical properties of Thin-film Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys., 47, 5248 (1976), etc. Examples of the MIM types are disclosed in C. A. Mead, "The Tunnel-emission Amplifier", J. Appl. Phys., 32, 646 (1961), etc. Examples of the SCE types are disclosed in M. I. Elinson, Radio Eng. Electron Phys., 10 (1965), etc.
The SCE types utilize a phenomenon in which emission of electrons takes place by flowing electric currents in parallel to film surface of a thin film with a small area, formed on a substrate. These surface conduction electron emitting devices include those employing an SnO.sub.2 film as reported by Elinson et al. mentioned above, those employing an Au thin film as reported in G. Dittmer, "Thin Solid Films", 9, 317 (1972), those employing an In.sub.2 O.sub.3 /SnO.sub.2 film as reported in M. Hartwell & C. G. Fonstad, "IEEE Trans. ED Conf.",519 (1975), and those comprising a carbon thin film as reported in Hisashi Araki et al., "SHINKU (Vacuum)", Vol. 26, No. 1, page 22 (1983).
The present applicants have already proposed a thin type image display apparatus making use of a surface conduction electron emitting device (SCE) as an electron emitting source, where light is emitted by irradiating a fluorescent material with accelerated electron beams to display an image (Japanese Patent Application Laid-open No. 3-261024).
An example of the construction of the flat-plate type image display apparatus employing the cold-cathode electron emitting source as mentioned above will be described below with reference to the accompanying drawing.
FIG. 6 illustrates such a flat-plate type image display apparatus. As shown in FIG. 6, the apparatus is, successively from the back to the front, comprised of a rear plate 61 provided with a plane cold cathode 62, a first spacer 63, an electrode substrate 65 provided with a control electrode 64 that controls electron beam currents and a focusing electrode 66 that focuses electron beams on a fluorescent screen and having apertures at predetermined intervals, a second spacer 67, and a face plate 69 provided with a fluorescent material 68 and an electron beam accelerating electrode. The above constituent members are sealed with a low-melting glass frit at their ends so that the inside components are enveloped in vacuum. Vacuum extraction is carried out through a vacuum exhaust tube 12.
Here, the above first spacer 63 and second spacer 67 are spacers made of electrical insulators, for which glass, ceramic or the like is used. Glass, ceramic or the like is also used in the electrode substrate 65, on both sides of which the control electrode 64 and the focusing electrode 66 are respectively formed by screen printing. In the first and second spacers and the electrode substrate, apertures 65-1, slits 63-1 and 67-1 parallel to plane cold electrodes, and a vacuum exhaust slit 65-2 are respectively formed. These apertures and slit can be made by etching or mechanical working.
The first spacer 63 and the electrode substrate 65 each have a thickness of about 1.0 mm, and the second spacer 67 has a thickness of about 3 to 5 mm for the purpose of a prevention for the discharge between upper and lower substrates.
In the display window board 69, an electron beam accelerating electrode (not shown) and on this electrode an R-G-B fluorescent material and a metal back layer (not shown) are formed. To the accelerating electrode, for example a high voltage of 10 kV to 20 kV is applied. Its inside is thus constructed.
In the display apparatus as described above that requires vacuum extraction of the inside of the apparatus, gettering is carried out in order to maintain the vacuum in the apparatus, which is a process in which a getter serving as a vacuum maintenance member is flashed to deposit the gettering material to the inner surfaces of the image display apparatus.
In this case, the getter is usually provided in the apparatus at its part having no influence on image display, e.g., on its rear plate or side walls. The getter is commonly comprised of gettering material and a metal tube, which gettering material is mainly composed of Ba, and with which gettering material the inside of a metal tube made open in part is filled. The metal tube may have a linear shape or the shape of a ring. The getter is flashed by induction heating or electric heating to deposit the gettering material to the inner surfaces of an image display apparatus, and adsorbs remaining gas to maintain the vacuum in the apparatus.
The pressure inside the image display apparatus whose vacuum exhaust tube has been closed and having the getter is commonly expressed as follows: ##EQU1## wherein V is a volume in which the image display apparatus forms a vacuum, p is a pressure of the space in which the image display apparatus forms the vacuum, Q(t) is a quantity of gas released from members inside the image display apparatus, and G(t) is an exhaust velocity of the getter. After adequate lapse of time, the inside pressure becomes substantially constant, and its value is given as follows: ##EQU2## The quantity of the gas released from members inside the image display apparatus is proportional to the surface area of the members, and the exhaust velocity of the getter is also proportional to the surface area of the flashed gettering material. It therefore is necessary for the gettering material to have a larger area in order to maintain a vacuum in an image display apparatus having a larger area.
The gettering, however, has been hitherto involved in the following problems.
(1) When the getter as a vacuum maintenance member is deposited on the inner surfaces of the image display apparatus, it is deposited also on members concerned in image display, for example the cold cathode and the control electrodes, resulting in a great deterioration of image quality of display images. In particular, in the case of the cold cathode of the surface conduction type electron emitting devices or the field emission type electron emitting devices, the state of its surface has a great influence on the emission of electrons, and hence is very greatly affected by the deposition of the gettering material. Accordingly, when these electron emitting devices are used, it is necessary to give a space large enough not to be concerned in image display so that such an influence can be avoided as far as possible. This, however, results in a large size of the image display apparatus. PA1 (2) Making the screen larger results in an increase in the inner surface areas inside the image display apparatus and also results in an increase in the area over which the gettering material necessary for maintaining the vacuum inside the image display apparatus is deposited. Hence, the position at which the getter is provided must be taken into account so that the gettering material can be well spread over the necessary inner surfaces of the apparatus. However, partly because of the above problem (1), it has been difficult to ensure a proper position at which the getter is provided.